1. Field of the Invention
This invention relates to an interferometer having a multi-mode semiconductor laser used as a light source and, more particularly, to an interferometer which irradiates a moving diffraction grating with laser light, forms interference light from diffracted light emerging from the diffraction grating and measures the interference light.
2. Description of the Prior Art
A type of encoder is well known which detects the displacement of a scale by superposing diffracted lights emerging from a diffraction grating formed on the scale to form interference light and converting the same into an electrical signal. The periodic signal obtained by this type of encoder has improved resolution compared with encoders using the ordinary combination of a main scale and an index scale. However, to use interference light for detection of displacement, the conditions for the occurrence of the interference of light must be established. In general, if the difference between the optical paths of two light beams to be made to interfere with each other (the difference between the times taken to reach the interfering position from the light source) is zero, the intensity of the interference signal is maximized irrespective of the type of light source. However, as the width of the spectrum of the light source (spectral width) becomes wider, the intensity of interference light decreases abruptly due to the slight difference between the optical paths of the two light beams. Lasers which emit light of very small spectral widths have, therefore, been used as light sources for interferometers, including the above-mentioned type of encoder, utilizing interference light. Specifically, semiconductor lasers have many advantages, for example, that of being reduced in size and requiring a reduced driving current, and they therefore contribute to reductions in the overall size and the production cost of interferometers if they are incorporated therein as light sources.
However, the following problems are encountered when semiconductor lasers are used as light sources for interferometers.
Semiconductor lasers include a single mode laser having one oscillation spectrum mode and a multi-mode laser having a plurality of oscillation spectrum modes. A single mode laser has improved interference performance (has a larger coherence length) but changes discontinuously with changes in operating temperature and driving current so that the phase of the interference signal changes discontinuously, resulting in errors in detecting changes in the intensity of interference light by counting periodic signals corresponding to these changes. For prevention of this phenomenon, it is necessary to add a system for accurately controlling the operating temperature and the driving current. The provision of such means necessarily makes the interferometer larger, more complicated and more expensive.
On the other hand, multi-mode lasers ordinarily have shorter coherence lengths and various oscillation spectra. Their interference performance is therefore unstable. For this reason, they cannot be easily utilized in interferometers.